876
FXUS62 KGSP 270621
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
221 AM EDT Fri Sep 27 2024

.SYNOPSIS...
Tropical Cyclone Helene will move quickly north through Georgia
through the overnight hours, bringing continued heavy rainfall,
significant flooding and strong winds through Friday morning. After
the storm departs Friday afternoon, expect a calmer weekend with
temperatures near seasonal normals.

&&

.NEAR TERM /THROUGH TODAY/...
***A HISTORIC RAINFALL EVENT IS UNDERWAY FOR THE SOUTHERN
APPALACHIANS AND VICINITY WITH WIDESPREAD/LOCALLY CATASTROPHIC FLASH
FLOODING EXPECTED***

As of 150 am: Moderate to heavy rain continues across the forecast
area early this morning. Rainfall rates associated with the ongoing
activity are relatively tame...generally .25-.5" per hour...and are
therefore not necessarily exacerbating ongoing flooding to any
appreciable degree. However, the center of Hurricane Helene is
currently just west of Alma, GA...and the cyclone...which is already
moving quickly at 25-30 mph...is forecast to accelerate as it moves
north across Georgia through the morning. Tropical rainbands...
likely producing rainfall rates of 1-2" per hour...will begin to
rotate across the CWA during the 08-12Z time frame...and this is
when the flash flood situation...which is already severe in some
areas...will become potentially catastrophic...especially in
locations that received the brunt of the predecessor event over the
past 36 hours...particularly the counties encompassing the Blue
Ridge escarpment...note that this also includes the city of
Asheville and vicinity. It`s very possible that ongoing warnings
will be upgraded to EMERGENCY designation in these areas. Reservoirs
along the Catawba River chain are also running high and these  could
be breached as enuf water may not be able to be released beforehand.
Please remain vigilant to any warnings issued this morning as
conditions can worsen rather quickly. Additional rainfall amounts of
3-6 inches are expected across all but the southern and eastern
fringe of the CWA (where 1.5-3" are expected). This additional
rainfall will result in at least "run-of-the-mill" flash flooding
across much of the remainder of the CWA.

A Tornado Watch remains in effect for the area from Greenwood to
Union SC to the Charlotte Metro area through 12Z Friday. Needless to
say, the low level and deep layer shear will remain more than
adequate as Helene makes its approach from the south through the
overnight hours. While the thermodynamic environment is not
particularly conducive for rotating updrafts at this time...sbCAPE
of 300-500 J/kg or so is poised just southeast of the area. An
impressive rotational feature was recently observed within this air
mass across the SC Midlands, so as the accelerating E/SE flow
potentially brings some of this more buoyant air into the southern
fringe of the CWA, the tornado threat is expected to ramp up a bit
in the Watch area toward daybreak.  The tornado threat may continue
through late Friday morning if the RAP and HRRR are correct, as they
bring a plume of 500 J/kg CAPE into the area along/E of I-77 as the
main circulation of Helene lifts northward in the morning.

As to the wind...no change to earlier thinking. The leading edge
of the significant wind gust potential will lift north across the
region from roughly 08Z to 11Z, maximizing from 10Z to 14Z south to
north. Indications of strong tropical storm force gusts continue in
the most recent guidance. Outside the mtns, there is some potential
that a very shallow stable boundary layer will keep the strongest
gusts from reaching the surface, but there will still be frequent
50kt gusts in many locations. The mountains will have no such
protection. It seems likely that peaks and ridgetops will get
frequent gusts to hurricane force. These winds will combine with
saturated soils and down widespread swaths of trees, producing
structural damage and knocking out power for many communities.

The system will continue to advance north of the forecast area late
Friday morning thru the afternoon. Improving weather conditions are
in store Friday afternoon as the column dries quickly from above,
however, the impacts and effects of this system will last thru
the weekend.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...
As of 200 PM Thu: By Friday evening, Helene`s center will be
northwest of the Appalachians and interacting with the lower OH
Valley upper low. The CWA should be pretty much entirely in the dry
slot of the transitioning system, but the low will be positioned to
allow vort lobes to swing across the mountains, and we accordingly
keep a small PoP generally west of I-26 and northwest of I-85. 850mb
flow will continue to be southerly/upslope, and although the dry
slot will bear PWATs well under an inch initially, those values
may rebound toward daybreak Saturday. That said, the moisture
Fri night will be beneath a deep subsidence inversion and rainfall
rates/amounts are expected to pale in comparison to those earlier in
the day, so it is unlikely there will be any new flood or landslide
impacts, except perhaps on slopes made unstable by the earlier
rainfall. Gusty winds will continue particularly in higher terrain
overnight, where frequent gusts of 25-30 mph will remain possible
into Saturday morning; shallow instability may still tap into 20+
kt winds in some lower elevations over the western half of the area.

By Saturday morning, the sfc/upper low will be stacked, and
Helene should be no longer distinguishable from a regular inland
closed low. Heights will continue to rise early Saturday but then
fall a bit from the west as another vort lobe rotates over the
CWA. Convective layer deepens and appears a little more viable
in our west, so PoPs expand across the mountains again. PWATs may
rebound over 1.5 inches but CAPE does look to remain no more than
a couple hundred joules. Effective shear looks unlikely to support
a severe threat; locally heavy rain could redevelop as well as a
localized flood risk. WPC maintains Marginal Risk excessive rain
outlook for the mountains on Saturday, which seems reasonable in
light of antecedent conditions. All of the above factors will be
in play again Sunday, just shifted slightly further north/east,
so PoPs end up similar but expanded slightly in those directions.

Temps will be mild, slightly above normal for the Piedmont each
afternoon, and slightly below over the cloudier mountains.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 230 PM Thu: Sunday night the upper low looks to begin to
rejoin the westerlies, at least partially, although the process is
slow and somewhat unsettled weather will continue through Monday
if not Tuesday. Some guidance depicts pattern almost getting to
a Rex block, with the remnant upper low beneath the eastern CONUS
ridge, but it looks somewhat transient. We retain small PoPs over
the northwest half of the CWA both days due to the upper low, but
with less potential enhancement from vort lobes. Rainfall rates are
likely to be light and currently appear unlikely to hinder recovery
efforts. Temperatures trend slightly warmer through Wednesday,
but remain within a couple degrees of normal in all locations.

&&

.AVIATION /07Z FRIDAY THROUGH TUESDAY/...
At KCLT and elsewhere: Flight conditions will deteriorate rapidly
over the next 3-6 hours as Hurricane Helene moves rapidly north
through Georgia. The main concern will be winds...as gusts in the
45-55 kts range...primarily from the E/SE are looking increasingly
likely from around 09-15Z. Some gusts as high as 60 kts are possible
at the upstate SC terminals and KAVL. Strong LLWS as high as 70 kts
will also develop at all sites later this morning. Otherwise, IFR
cigs are expected to prevail, although these will occasionally mix
up to MVFR as heavier rainfall rates/convective elements pass over
the terminals. Isolated TS are also possible...with VCTS included at
m most sites. Otherwise, winds will steadily diminish and tropical
rain bands taper off from late morning through early afternoon. Low
cigs are expected to become VFR by late afternoon, while gusts are
likely to subside after 00Z.

Outlook: Isolated to sct mainly diurnal/mainly mountain convection
(primarily showers) will be possible thru early next week. Morning
fog/low stratus will be possible each day, mainly in the mountain
valleys.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch until 2 PM EDT this afternoon for GAZ010-017-018-
     026-028-029.
     Tropical Storm Warning for GAZ010-017-018-026-028-029.
NC...Flood Watch until 2 PM EDT this afternoon for NCZ033-035>037-
     048>053-056>059-062>065-068>072-082-501>510.
     Tropical Storm Warning for NCZ033-048>053-056-058-059-062>065-
     068>071-082-501>510.
     Wind Advisory until 2 PM EDT this afternoon for NCZ035>037-057-
     072.
SC...Flood Watch until 2 PM EDT this afternoon for SCZ008>014-019-
     101>109.
     Tropical Storm Warning for SCZ008>014-019-101>109.
&&

$$

SYNOPSIS...PM
NEAR TERM...JDL/PM/SBK
SHORT TERM...Wimberley
LONG TERM...Wimberley
AVIATION...JDL